DESCRIPTION (Applicant's Abstract): The overall goal of this independent research project is to improve the understanding of the proximal and distal causes of individual differences in emotional reactivity that define affective style. The proximal influences will focus on affective chronometry the temporal course of emotional responding. The investigators propose that an important source of individual differences in emotional responding arises from difference in chronometry. For example, some individuals show more rapid recovery from negative emotional events while others show more persistence of negative emotional events while others show more persistence of negative emotional reactions following a negative challenge. Such individual differences in affective chronometry are further proposed to be a major determinant of certain forms of psychopathology. A further goal of this project will therefore be to examine in patients with affective disorders the chronometric measures that were developed and characterized in preceding studies in normal human subjects. Both the studies of affective chronometry in normative and patient populations will be extended to include the characterization of the neural correlates using functional neuroimaging techniques (FDG-PET and fMRI). Three studies are proposed to address the goals of this independent research project. Study 1 would use both emotion-modulated startle and classical aversive conditioning to characterize effective chronometry in normal human subjects and to define the behavioral and electrophysiological correlates. Study 2 would use FDG-PET and fMRI to define the neural correlates of affective chronometry by comparison of three groups representing the phenotypic extremes of the trait. Study 3 would characterize affective chronometry in depressed patients and use PET and fMRI to define the neural correlates of altered affective chronometry in depression. The distal causes of affective chronometry would also be examined by continuing the ongoing twin studies to define the genetic influences on brain electrical activity and autonomic activity that have been associated with affective style.